The impact of tillage intensity on the recruitment of false cleavers (Galium spurium L.)

By altering the microsite conditions, including soil temperature and soil moisture, tillage intensity can influence the germination and emergence of seeds.False cleavers (Galium spurium L.) recruitment may be reduced when weed seeds remain on or near the soil surface.Tillage affects the placement of weed seeds in the soil.False cleavers infestations may be limited by the intensity of tillage used in a given management system.In the summer of 2001 and2002, freld experiments were conducted to investigate the impact oftillage intensity on the recruitment of false cleavers.Experiments were conducted at two locations, and considered three tillage treatments; no additional tillage, spring and fall tillage and fall tillage only.Spring emergence counts indicated that recruitment was significantly greater in tilled treatments relative to untilled treatments in three of four site years.Spring gravimetric soil moisture measured from 0- 5cm did not vary significantly between the tillage treatments at either location, in either year.Spring soil temperature measured at 4cm was significantly different between tillage treatments in three of four site years.However, actual temperature differences recorded between treatments were not regarded as being biologically significant.In the spring of 2002, additional measurements were included to investigate the variation in soil ABSTRACT temperature and moisture at various soil depths.Temperature amplitude was greater at the 1cm verses the 4cm depth.This consisted of both greater maximum temperatures and lower minimum temperatures at lcm verses 4cm.Volumetric soil moisture levels also varied with depth (0-2,2-4,4-6cm), showing a tendency for the soil surface layer to be lx driest.Thus, temperature and moisture differences between tillage treatments did not appear to be controlling the spring false cleavers recruitment.Rather, differences in soil temperature and moisture experienced by false cleavers as a result of distinct seed placement in tilled verses untilled treatments was influencing spring false cleavers recruitment.In the untilled treatments seeds were concentrated on or near the soil surface.As such, they were exposed to extreme temperature and moisture conditions and to solar radiation.False cleavers germination is inhibited by the prolonged exposure to light.False cleavers seedling recruitment in the fall was very low for all tillage treatments.Extremely low moisture conditions may have led to the reduced fall recruitment.The low fall recruitment was also a reflection of the summer annual growth habit of the seed population used in our experiment.No fall emergence counts were done in2002In North America "cleavers" is the common name used to describe two species; Galium aparine L. (True cleavers) andGalium spurium L. (False cleavers).Both of these species are members of the Rubiaceae or Madder family, and are currently listed as a class 2 primary noxious weed according to the Canada Seed Act: Weed Seed Order (SOR/86-836) (Anonymous 1986).It is believed that in North America, the predominant species in agricultural fields is Galium spurium.Based on a relative abundance index, cleavers was ranked as one of the top 20 weed species in the 2002Manitoba Weed Survey (Leeson et aI.2002).Cleavers is considered a competitive weed, leading to yield losses and reduced crop quality (Malik and Vanden Born L987a).Initially, in Western Canadathe establishment of cleavers appeared to coincide with an increase in the acreage of land seeded to canola (Brassica napus L.).This was largely due to the absence of effective chemical control for cleavers in canola and crop seed contamination.However, since the advent of herbicide resistant canola varieties, control of many weed species, including cleavers, has been enhanced in this crop.As a result, it was believed that cleavers would no longer be a significant pest on the Canadian prairies.However, the relative abundance of cleavers in Manitoba has not changed from 1997 to 2002, suggesting that the introduction of herbicide tolerant crop varieties is not significantly reducing the occurrence of cleavers (Leeson et al. 2002).In addition to the natural hardiness of this weed, control is further complicated by the development of herbicide resistance.In Canada, a biotype of cleavers exhibiting multiple resistance to Group 2 (ALS Inhibitors) and Group 4 (Synthetic Auxins) herbicides was discovered in Alberta in 1996 (Hall et al. 1998).The presence of GENERAL INTRODUCTION herbicide resistant biotypes within Canada reinforces the importance of identifying nonchemical methods forthe control of cleavers.Cleavers recruitment is sensitive to soil moisture level and seeding depth (Boyd and Van Acker, 2003).Responses are partially related to the inhibitory effect of light exposure on cleavers seed germination.Even at very low light intensities recruitment is reduced when cleavers seeds are exposed to light after 36hours of imbibition (Malik and Vanden Born 1987c).In the freld, tillage moves weed seeds within the soil profile.The ultimate placement of the seeds is dependant on the intensity of the tillage operation used.Where no tillage is applied, weed seeds are concentrated on, or near, the soil surface.Where soils are tilled, the weed seeds tend to concentrate near the lower region of the tillage zone.Differences in placement will affect the conditions that weed seeds are exposed to.Tillage may also directly affect the conditions of the soil environment by altering the soil physical properties and surface residue cover.These changes in soil conditions, specifically soil temperature and soil moisture, may also affect weed seedling recruitment.Thus, tillage may impact the conditions experienced by weed seeds either directly by affecting soil properties or indirectly by changing the position of seeds within the soil profile.Varying the intensity of tillage may impact the establishment and success of cleavers on agricultural fields.

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